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- Frequently Asked Questions (FAQS);faqs.475
-
-
-
- 1.22. Both women are white; the one whose house this takes place in is
- single. A black friend of the other woman, the one who goes into the
- bathroom, was recently killed, reportedly by the KKK. The woman who goes
- into the bathroom discovers a bloodstained KKK robe in the other's laundry
- hamper, picks up a nail file from the medicine cabinet (or some other
- impromptu weapon), and goes out and kills the other.
- 1.22a. Variant: A man goes to hang his coat and realises he will die that
- day. Answer: The man (who is black) has car trouble and is in need of a
- telephone. He asks at the nearest house and on being invited in goes to
- hang his coat, whereupon he notices the white robes of the Ku Klux Klan in
- the closet. (from Bernd Wechner)
-
- 1.23. He is in a hotel, and is unable to sleep because the man in the
- adjacent room is snoring. He calls the room next door (from his own
- room number he can easily figure out his neighbor's, and from the room
- number, the telephone number). The snorer wakes up, answers the phone.
- The first man hangs up without saying anything and goes to sleep before
- the snorer gets back to sleep and starts snoring again.
- 1.23a. Slightly variant answer: It's a next-door neighbor in an apartment
- building who's snoring, rather than in a hotel. The caller thus knows his
- neighbor and the phone number.
-
- 1.24. It's the man's fiftieth birthday, and in celebration of this he
- plans to kill his wife, then take the money he's embezzled and move on to
- a new life in another state. His wife takes him out to dinner; afterward,
- on their front step, he kills her. He opens the door, dragging her body
- in with him, and all the lights suddenly turn on and a group of his
- friends shout "Surprise!" He kills himself. (Note that the whole first
- part, including the motive, isn't really necessary; it was just part of
- the original story.)
-
- 1.25. Abel is a prince of the island nation that he landed on. A cruel
- and warlike prince, he waged many land and naval battles along with his
- father the king. In one naval encounter, their ship sank, the king died,
- and the prince swam to a deserted island where he spent several months
- building a raft or small boat. In the meantime, a regent was appointed to
- the island nation, and he brought peace and prosperity. When Prince Abel
- returned to his kingdom, Cain (a native fisherman) realized that the peace
- of the land would only be maintained if Abel did not reascend to his
- throne, and killed the prince (with a piece of driftwood or some other
- impromptu weapon).
-
- 1.26. The drinks contain poisoned ice cubes; one man drinks slowly,
- giving them time to melt, while the other drinks quickly and thus doesn't
- get much of the poison. The fact that they drink at different speeds
- could be added to the statement, possibly along with red herrings such as
- saying that one of the men is big and burly and the other short and thin.
-
- 1.27. Joe is a kid who goes trick-or-treating for Halloween.
-
- 1.28. He's a smuggler. On the first cruise, someone brings the
- contraband to his cabin, and he hides it in an air conditioning duct.
- Returning to the U.S., he leaves without the contraband, and so passes
- through customs with no trouble. On the second trip, he has the same
- cabin on the same ship. Because it doesn't stop anywhere, he doesn't have
- to go through customs when he returns, so he gets the contraband off
- safely.
-
- 1.29. Hans and Fritz do everything right up until they're filling out a
- personal-information form and have to write down their birthdays. Fritz'
- birthday is, say, July 7, so he writes down 7/7/15. Hans, however, was
- born on, say, June 20, so he writes down 20/6/18 instead of what an
- American would write, 6/20/18. Note that this is only a problem because
- they *claim* to be returning Americans; as has been pointed out to me,
- there are lots of other nations which use the same date ordering.
-
- 1.30. Another WWII story. Greg is a German spy. His "friend" Tim is
- suspicious, so he plays a word-association game with him. When Tim says
- "The land of the free," Greg responds with "The home of the brave." Then
- Tim says "The terror of flight," and Greg says "The gloom of the grave."
- Any U.S. citizen knows the first verse of the national anthem, but only a
- spy would have memorized the third verse. (Why Tim knew the third verse
- is left as an exercise to the reader.)
-
- 1.31. The dead man was the driver in a hit-and-run acccident which
- paralyzed its victim. The victim did manage to get the license plate
- number of the car; now in a wheelchair, he eventually tracked down the
- driver and shot and killed him.
-
- 1.32. His home is a houseboat and he has run out of water while on an
- extended cruise.
- 1.32a. Variant wording: A man dies of thirst in his own home. This
- version goes more quickly because it gives more information; but it may be
- less likely to annoy people who think the original statement is too vague.
-
- 1.33. I'm told this is a true story. Windows in Paris at that time were
- apparently imperfectly flat; they could act as lenses. One particularly
- hot day, the sun shining in through such a window caused a woman's
- lingerie (which she was wearing at the time, awaiting her husband's
- return) to catch fire, and eventually the entire house caught and burned.
-
- 1.34. He's leaving a hospital after visiting his wife, who's on heavy
- life-support. When the power goes out, he knows she can't live without
- the life-support systems (he assumes that if the emergency backup
- generator were working, the elevator wouldn't lose power; this aspect
- isn't entirely satisfactory, so in a variant, the scene is at home rather
- than in a hospital).
- 1.34a. Variant: The music stops and a woman dies. Answer: The woman is
- confined in an iron lung, and the music is playing on her radio or stereo.
- The power goes out. (from Randy Whitaker) (See also #1.15a, #1.16, and
- #1.19e.)
-
- 1.35. A large man comes home to the penthouse apartment he shares with
- his beautiful young wife, taking the elevator up from the ground floor.
- He sees signs of lovemaking in the bedroom, and assumes that his wife is
- having an affair; her beau has presumably escaped down the stairs. The
- husband looks out the French windows and sees a good-looking man just
- leaving the main entrance of the building. The husband pushes the
- refrigerator out through the window onto the young man below. The husband
- dies of a heart attack from overexertion; the young man below dies from
- having a refrigerator fall on him; and the wife's boyfriend, who was
- hiding inside the refrigerator, also dies from the fall.
-
- 1.36. Let's say "she" is named Suzy, and "they" are named Harry and Jane.
- Harry is an elderly archaeologist who has found a very old skeleton, which
- he's dubbed "Jane" (a la "Lucy"). Suzy is a buyer for a museum; she's
- supposed to make some sort of purchase from Harry, so she invites him to
- have a business dinner with her (at a restaurant). When she calls to
- invite him, he keeps talking about "Jane," so Suzy assumes that Jane is
- his wife and says to bring her along. Harry, offended, calls Suzy's boss
- and complains; since Suzy should've known who Jane was, she gets fired.
-
- 1.37. The man is delivering a pardon, and the flicker of the lights
- indicates that the person to be pardoned has just been electrocuted.
-
- 1.38. The murderer sets the car on a slope above the hot dog stand where
- the victim works. He then wedges an ice block in the car to keep the
- brake pedal down, and puts the car in neutral, after which he flies to
- another city to avoid suspicion. It's a warm day; when the ice melts, the
- car rolls down the hill and strikes the hot dog man at his roadside stand,
- killing him.
-
- 1.39. There's a car wash on that corner. On rainy days, the rain reduces
- traction. On sunny days, water from the car wash has the same effect. If
- rain is threatening, though, the car wash gets little business and thus
- doesn't make the road wet, so I can take the corner faster.
-
- 1.40. The object she throws is a boomerang. It flies out, loops around,
- and comes back and hits her in the head, killing her. Boomerangs do not
- often return so close to the point from which they were thrown, but I
- believe it's possible for this to happen.
- 1.40a. Silly variant answer: She's in a submarine or spacecraft and
- throws a heavy object at the window, which breaks.
-
- 1.41. He is a passenger in an airplane and sees the bird get sucked into
- an engine at 20,000 feet.
-
- 1.42. They're the remains of a melted snowman.
-
- 1.43. One of the brothers (A) confesses to the murder. At his trial, his
- brother (B) is called as the only defense witness; B immediately
- confesses, in graphic detail, to having committed the crime. The defense
- lawyer refuses to have the trial stopped, and A is acquitted under the
- "reasonable doubt" clause. Immediately afterward, B goes on trial for the
- murder; A is called as the only defense witness and HE confesses. B is
- declared innocent; and though everyone knows that ONE of them did it, how
- can they tell who? Further, neither can be convicted of perjury until
- it's decided which of them did it... I don't know if that would actually
- work under our legal system, but someone else who heard the story said
- that his father was on the jury for a VERY similar case in New York some
- years ago. Mark Brader points out that the brothers might be convicted of
- conspiracy to commit perjury or to obstruct justice, or something of that
- kind.
-
- 1.44. He is a mail courier who delivers packages to the different foreign
- embassies in the United States. The land of an embassy belongs to the
- country of the embassy, not to the United States.
-
- 1.45. A man was shot during a robbery in his store one night. He
- staggered into the back room, where the telephone was, and called home,
- dialing by feel since he hadn't turned on the light. Once the call went
- through he gasped, "I'm at the store. I've been shot. Help!" or words to
- that effect. He set the phone down to await help, but none came; he'd
- treated the telephone pushbuttons like cash register numbers, when the
- arrangements of the numbers are upside down reflections of each other.
- The stranger he'd dialed had no way to know where "the store" was.
-
- 1.46. The dead man was playing Santa Claus, for whatever reason; he
- slipped while coming down the chimney and broke his neck.
- 1.46a. Variant answer: The dead man WAS Santa Claus. This moves the
- puzzle to section 2.
-
- 1.47. The man was struck by an object thrown from the roof of the Empire
- State Building. Originally I had the object being a penny, but several
- people suggested that a penny probably wouldn't be enough to penetrate
- someone's skull. Something aerodynamic and heavier, like a dart, was
- suggested, but I don't know how much mass would be required.
- 1.47a. Variant: A man is found dead outside a large marble building with
- three holes in him. Answer: The man was a paleontologist working with the
- Archaeological Research Institute. He was reviving a triceratops frozen
- in the ice age when it came to life and killed him. This couldn't
- possibly happen because triceratops didn't exist during the ice age.
- (from Peter R. Olpe)
-
- 1.48. The man died from eating a poisoned popsicle.
-
- 1.49. The man was a sword swallower in a carnival side-show. While he
- was practicing, someone tickled his throat with the feather, causing him
- to gag.
-
- 1.50. A mosquito bit me, and I swatted it when it later landed on my
- ceiling (so the blood is my own as well as the mosquito's).
-
- 1.51. The man is a lighthouse keeper. He turns off the light in the
- lighthouse and during the night a ship crashes on the rocks. Seeing this
- the next morning, the man realizes what he's done and commits suicide.
- 1.51a. Variant, similar to #1.15: The light goes out and a man dies.
- Answer: The lighthouse keeper uses his job as an alibi while he's
- elsewhere committing a crime, but the light goes out and a ship crashes,
- thereby disproving the alibi. The lighthouse keeper kills himself when he
- realizes his alibi is no good. (From Eric Wang)
- 1.51b. Variant answer to 1.51a: Someone else's alibi is disproven. (A
- man commits a heinous crime, claiming as his alibi that he was onboard a
- certain ship. When he learns that it was wrecked without reaching port
- safely, he realizes that his alibi is disproven and commits suicide to
- avoid being sent to prison.) (From Eric Wang)
-
- 1.52. They were skydiving. He broke his arm as he jumped from the plane
- by hitting it on the plane door; he couldn't reach his ripcord with his
- other arm. She pulled the ripcord for him.
- 1.52a. Sketch of variant answer: The ring was attached to the pin of a
- grenade that he was holding. Develop a situation from there.
-
- 1.53. The man is a travel agent. He had sold someone two tickets for an
- ocean voyage, one round-trip and one one-way. The last name of the man
- who bought the tickets is the same as the last name of the woman who
- "fell" overboard and drowned on the same voyage, which is the subject of
- the article he's reading.
-
- 1.54. The man is a beekeeper, and the bees attack en masse because they
- don't recognize his fragrance. Randy adds that this is based on something
- that actually happened to his grandfather, a beekeeper who was severely
- attacked by his bees when he used a new aftershave for the first time in 10
- or 20 years.
-
- 1.55. He is a guard / attendant in a leper colony. The letter (to him)
- tells him that he has contracted the disease. The key is the cigarette
- burning down between his fingers -- leprosy is fairly unique in killing off
- sensory nerves without destroying motor ability. (Randy was told this by
- Gary Haas and Chris Englehard)
-
- 1.56. The man was a famous artist. A woman who collected autographs saw
- him dining; after he left the restaurant, she purchased the check that he
- used to pay for the meal from the restaurant manager. The check was
- therefore never cashed, so the artist never paid for the meal.
-
- 1.57. The movie is at a drive-in theatre.
-
-
- Section 2: Double meanings, fictional settings, and miscellaneous others.
-
- 2.1. The man is a heroin addict, and has contracted AIDS by using an
- infected needle. In despair, he shoots himself up with an overdose,
- thereby committing suicide.
-
- 2.2. The man walks into a casino and goes to the craps table. He bets
- all the money he owns, and shoots craps. Since he is now broke, he
- becomes despondent and commits suicide.
-
- 2.3. Kids getting their pictures taken with Santa. I see #2.1, #2.2, and
- #2.3 as different enough from each other to merit separate numbers,
- although they all rely on the same basic gimmick of alternate meanings of
- the word "shoot."
-
- 2.4. It's the cabin of an airplane that crashed there because of the
- snowstorm.
- 2.4a. Variant wording: A cabin, on the side of a mountain, locked from
- the inside, is opened, and 30 people are found dead inside. They had
- plenty of food and water. (from Ron Carter)
-
- 2.5. He's a priest; he is marrying them to other people, not to himself.
-
- 2.6. The "island" is a traffic island.
-
- 2.7. A baseball game is going on. The base-runner sees the catcher
- waiting at home plate with the ball, and so decides to stay at third base
- to avoid being tagged out.
- 2.7a. Variant: Two men are in a field. One is wearing a mask. The other
- man is running towards him to avoid him. Answer: the same, but the
- catcher isn't right at home plate; the runner is trying to get home before
- the catcher can. (from Hal Lowery, by way of Chris Riley) This phrasing
- would allow the puzzle to migrate to section 1, but I don't like it as
- much.
-
- 2.8. The man is an astronaut out on a space walk.
-
- 2.9. The man was an amateur mechanic, the book is a Volkswagen service
- manual, the beetle is a car, and the pile of bricks is what the car fell
- off of.
-
- 2.10. The Eagle landed in the Sea of Tranquility and will likely remain
- there for the foreseeable future.
-
- 2.11. It's a wolf pack; they've killed and eaten (most of) the man.
-
- 2.12. The dead man is Superman; the rock is Green Kryptonite. Invent a
- reasonable scenario from there.
-
- 2.13. This is a post-holocaust scenario of some kind; for whatever
- reason, the man believes himself to be the last human on earth. He
- doesn't want to live by himself, so he jumps, just before the telephone
- rings... (of course, it could be a computer calling, but he has no way of
- knowing).
-
- 2.14. The one who looks around sees his own reflection in the window
- (it's dark outside), but not his companion's. Thus, he realizes the other
- is a vampire, and that he's going to be killed by him.
-
- 2.15. The "bicycles" are Bicycle playing cards; the man was cheating at
- cards, and when the extra card was found, he was killed by the other
- players.
- 2.15a. Variant: There are 53 bees instead of 53 bicycles. Answer: The
- same (Bee is another brand of playing cards).
- 2.15b. Variant: There are 51 instead of 53. Answer: Someone saw the guy
- conceal a card, and proved the deck was defective by turning it up and
- pointing out the missing ace. Or, the game was bridge, and the others
- noticed the cheating when the deal didn't come out even. The man had
- palmed an ace during the shuffle and meant to put it in his own hand
- during the deal, but muffed it. (both answers from Mark Brader)
-
- 2.16. A chess game; knight takes pawn.
- 2.16a. Variant: It's the year 860 A.D., at Camelot. Two priests are
- sitting in the castle's chapel. The queen attacks the king. The two
- priests rise, shake hands, and leave the room. Answer: The two priests
- are playing chess; one of them just mated by moving his queen. (from
- Ellen M. Sentovich)
- 2.16b. Variant: A black leader dies in Africa. Answer: The black leader
- is a chess king, and the game was played in Africa. (from Erick
- Brethenoux)
-
- 2.17. It's a model train set.
- 2.17a. Variant: The Orient Express is derailed and a kitten plays nearby.
- Answer: The Orient Express is a model train which has been left running
- unattended. The kitten has playfully derailed it. (from Bernd Wechner)
-
- 2.18. It's a game of Monopoly.
-
- 2.19. The sun is shining; there's no rain.
-
- 2.20. It's daytime; the sun is out.
-
- 2.21. Alice is a goldfish; Ted is a cat.
- 2.21a. A very common variant uses the names Romeo and Juliet instead, to
- further mislead audiences. For example: Romeo is looking down on Juliet's
- dead body, which is on the floor surrounded by water and broken glass.
- (from Adam Carlson)
- 2.21b. Minor variant: Tom and Jean lay dead in a puddle of water with
- broken pieces of glass and a baseball nearby. Answer: Tom and Jean are both
- fish; it was a baseball, rather than a cat, that broke their tank. (from
- Mike Reymond)
-
- 2.22. Friday is a horse.
- 2.22a. Variant with the same basic gimmick: A woman comes home, sees
- Spaghetti on the wall and kills her husband. Answer: Spaghetti was the
- name of her pet dog. Her husband had it stuffed and mounted after it made
- a mess on his rug. (Simon Travaglia original)
-
- 2.23. Bruce is a horse.
-
- 2.24. Should be done orally; the envelope is an evelope of dye, and she's
- dying some cloth, but it sounds like "opens an envelope and dies" if said
- out loud.
-
- 2.25. The native chief asked him, "What is the third baseman's name in
- the Abbot and Costello routine 'Who's on First'?" The man, who had no
- idea, said "I don't know," the correct answer. However, he was a major
- smartass, so if he had known the answer he would have pointed out that
- What was the SECOND baseman's name. The chief, being quite humorless,
- would have executed him on the spot. This is fairly silly, but I like it
- too much to remove it from the list.
-
- 2.26. The men have gone spelunking and have taken an Igloo cooler with
- them so they can have a picnic down in the caves. They cleverly used dry
- ice to keep their beer cold, not realizing that as the dry ice sublimed
- (went from solid state to vapor state) it would push the lighter oxygen
- out of the cave and they would suffocate.
-
- ==> logic/smullyan/black.hat.p <==
- Three logicians, A, B, and C, are wearing hats, which they know are either
- black or white but not all white. A can see the hats of B and C; B can see
- the hats of A and C; C is blind. Each is asked in turn if they know the color
- of their own hat. The answers are:
- A: "No."
- B: "No."
- C: "Yes."
- What color is C's hat and how does she know?
-
- ==> logic/smullyan/black.hat.s <==
- A must see at least one black hat, or she would know that her hat is black
- since they are not all white. B also must see at least one black hat, and
- further, that hat had to be on C, otherwise she would know that her
- hat was black (since she knows A saw at least one black hat). So C knows
- that her hat is black, without even seeing the others' hats.
-
- ==> logic/smullyan/fork.three.men.p <==
- Three men stand at a fork in the road. One fork leads to Someplaceorother;
- the other fork leads to Nowheresville. One of these people always answers
- the truth to any yes/no question which is asked of him. The other always
- lies when asked any yes/no question. The third person randomly lies and
- tells the truth. Each man is known to the others, but not to you.
- What is the least number of yes/no questions you can ask of these men and
- pick the road to Someplaceorother?
-
- ==> logic/smullyan/fork.three.men.s <==
- It is clear that you must ask at least two questions, since you might be
- asking the first one of the randomizer and there is nothing you can tell
- from his answers.
-
- Start by asking A "Is B more likely to tell the truth than C?"
-
- If he answers "yes", then:
- If A is truthteller, B is randomizer, C is liar.
- If A is liar, B is randomizer, C is truthteller.
- If A is randomizer, C is truthteller or liar.
-
- If he answers "no", then:
- If A is truthteller, B is liar, C is randomizer.
- If A is liar, B is truthteller, C is randomizer.
- If A is randomizer, B is truthteller or liar.
-
- In either case, we now know somebody (C or B, respectively) who is either
- a truthteller or liar. Now, use the technique for finding information from
- a truthteller/liar, viz.:
-
- You ask him the following question: "If I were to ask a person of the opposite
- type to yourself if the left fork leads to Someplacerother, would he say yes?"
-
- If the person asked is a truthteller, he will tell you what a liar would
- say, which would be the wrong information. If the person asked is a liar,
- he will either tell you what a liar would say, or he will lie about what a
- truthteller would say. In either case, he will report the wrong information.
- If the answer is yes, take the right fork, if no take the left fork.
-
- ==> logic/smullyan/fork.two.men.p <==
- Two men stand at a fork in the road. One fork leads to Someplaceorother; the
- other fork leads to Nowheresville. One of these people always answers the
- truth to any yes/no question which is asked of him. The other always lies
- when asked any yes/no question. By asking one yes/no question, can you
- determine the road to Someplaceorother?
-
- ==> logic/smullyan/fork.two.men.s <==
- The question to ask is: "Will the other person say the right fork leads to
- Someplaceorother?" If the person asked says yes, then take the left fork,
- else take the right fork.
-
- If the person asked is the truthteller, then he correctly reports that the
- liar will misinform you about the right fork. If he is the liar, then he
- lies about what the truthteller will say. Either way, you should go the
- opposite direction from the way that the person asked says the other person
- will answer.
-
- The fact that there are two is a red herring - you only need one of
- either type. You ask him the following question: "If I were to ask a
- person of the opposite type to yourself if the left fork leads to
- Someplacerother, would he say yes?"
-
- If the person asked is a truthteller, he will tell you what a liar would
- say, which would be the wrong information. If the person asked is a liar,
- he will either tell you what a liar would say, or he will lie about what a
- truthteller would say. In either case, he will report the wrong information.
- If the answer is yes, take the right fork, if no take the left fork.
-
- This solution also removes the problem that the men may not know the
- other's identity.
-
- It is possible, of course, that the liars are malicious, and they will tell
- the truth if they figure out that you are trying to trick them.
-
-
- ==> logic/smullyan/integers.p <==
- Two logicians place cards on their foreheads so that what is written on the
- card is visible only to the other logician. Consecutive positive integers
- have been written on the cards. The following conversation ensues:
- A: "I don't know my number."
- B: "I don't know my number."
- A: "I don't know my number."
- B: "I don't know my number."
- ... n statements of ignorance later ...
- A or B: "I know my number."
- What is on the card and how does the logician know it?
-
- ==> logic/smullyan/integers.s <==
- If A saw 1, she would know that she had 2, and would say so. Therefore,
- A did not see 1. A says "I don't know my number."
- If B saw 2, she would know that she had 3, since she knows that A did not see
- 1, so B did not see 1 or 2. B says "I don't know my number."
- If A saw 3, she would know that she had 4, since she knows that B did not
- see 1 or 2, so A did not see 1, 2 or 3. A says "I don't know my number."
- If B saw 4, she would know that she had 5, since she knows that A did not
- see 1, 2 or 3, so B did not see 1, 2, 3 or 4. B says "I don't know my number."
- ... n statements of ignorance later ...
- If X saw n, she would know that she had n + 1, since she knows that ~X did not
- see 1 ... n - 1, so X did see n. X says "I know my number."
-
- And the number in n + 1.
-
- ==> logic/smullyan/liars.et.al.p <==
- Of a group of n men, some always lie, some never lie, and the rest sometimes
- lie. They each know which is which. You must determine the identity of each
- man by asking the least number of yes-or-no questions.
-
- ==> logic/smullyan/liars.et.al.s <==
- The real problem is to isolate the sometimes liars.
-
- Consider the case of three men:
- Ask man 1: "Does man 2 lie more than 3?"
- If the answer is yes, then man 2 cannot be the sometimes liar.
- Proof by analyzing the cases:
- Case 1: Man 2 is not the sometimes liar.
- Case 2: Man 2 is the sometimes liar, man 1 is the truth teller, and man 3 is
- the liar. Then man 1 would not say that man 2 lies more than man 3.
- Case 3: Man 2 is the sometimes liar, man 3 is the truth teller, and man 1 is
- the liar. Then man 1 would not say that man 2 lies more than man 3.
- QED.
- Similarly, if the answer is no, then man 3 cannot be the sometimes liar.
- Now ask the symmetric question of whichever man has been eliminated as the
- sometimes liar. The answer will now allow you to determine the identity
- of the sometimes liar. To determine the identity of the two remaining men, ask
- some question like "Does 1=1?" which is always true.
-
- This is not the only way to solve this problem. You could have asked the
- question which is always true (or false) second, which would now establish
- the identity of either the liar or the truth teller. Then ask the third
- question of this man to find out which of the other two is the sometimes
- liar.
-
- This problem requires three questions, whether or not they are yes-or-no
- questions. In order to identify all three men, you must identify the
- sometimes liar. You cannot identify the sometimes liar in one question
- since you may be asking it of the sometimes liar, and any answer from him
- conveys no information at all. Therefore at least two questions are
- necessary to identify the sometimes liar. Once the sometimes liar is
- identified, you still need one more question at least to identify the
- remaining men. Therefore, three questions are required.
-
- Suppose we have two truth-tellers, two liars, and two randomizers.
- The answer is 8. A proof follows.
-
- For brevity, "T" means truth-teller, "L" liar, "R" randomizer, "P" predictable
- (either T or L). Define a _pattern_ to be one of the C(6,2)=15 permutations
- of RRPPPP (each of which has C(4,2)=6 interpretations of the Ps as 2 Ts and 2
- Ls). For any question Q, let !Q denote the question "If I were to ask you Q,
- would you answer Yes?". Note that question !Q directed toward any P will
- yield a truthful answer to question Q; in other words, a "Yes" answer to !Q
- means that either Q is true or the respondent is an R, whereas "No" means that
- either Q is false or the respondent is an R.
-
- Ask #1, !"Are both Rs in the set {#2, #3, #4}?". "No" implies that at most
- one of {#2, #3, #4} is an R. "Yes" implies that at most one of {#2, #5, #6}
- is an R. Without loss of generality, assume the former.
-